1. Field of the Invention
The present invention relates to a droplet discharging apparatus for forming an image by discharging droplets of a recording fluid, an image forming apparatus including the droplet discharging apparatus, and a bubble separating method implemented in the droplet discharging apparatus and the image forming apparatus.
2. Description of the Related Art
Image forming apparatuses include printers, facsimile machines, copy machines, plotters, and multifunction peripherals having multiple image forming functions. As an example of an image forming apparatus of a fluid-discharging recording type, an inkjet recording apparatus is known. In the inkjet recording apparatus, an image is formed (or recorded, transferred, or printed, for example) by discharging droplets of a recording fluid, such as ink, from the recording head onto a recording medium, such as a sheet of paper, as the recording medium is transported. The recording medium is not limited to a sheet of paper but may include various forms and materials to which the recording fluid can adhere, such as an OHP sheet.
There are two types of the image forming apparatus of the fluid-discharging recording type: a serial-type image forming apparatus that forms an image by discharging the droplets while the recording head is moved in a main-scan direction, and a line-type image forming apparatus that forms an image by discharging the droplets without moving the recording head.
The recording medium may include paper, thread, fibers, cloth, metal, plastics, glass, wood, and ceramics. The “image” herein may include anything resulting from the landing of the droplets of recording fluid on the recording medium. The “image” may include an image having no apparent meaning, such as a random pattern, as well as an image with some meaning, such as characters or figures. The recording fluid is not limited to ink but may include a DNA sample, a resist, or a pattern material. The “image” herein is not limited to two-dimensional images but may refer to an image formed on a three-dimensional object, or even a three-dimensional image.
Typically, an image forming apparatus of the droplet discharging system includes a recording head that discharges droplets of recording fluid; a detachable first liquid containing unit storing the fluid supplied to the recording head; and a replaceable second liquid containing unit capable of temporarily containing the fluid supplied from the first liquid containing unit via a fluid supply channel, and also storing air that may enter into the fluid supply channel from the outside. The second liquid containing unit may include a negative-pressure generating unit for creating a reduced-pressure by using the deformation of a flexible film caused by an internal spring.
The second liquid containing unit and the recording head may or may not be filled with the recording fluid in advance. When the second liquid containing unit or the recording head is filled with the recording fluid, fluid components may coagulate around the nozzles of the recording head during a long storage period, possibly causing a defective discharge operation. When the second liquid containing unit or the recording head is not filled with the recording fluid, bubbles may remain inside a fluid chamber of the recording head following the initial loading of recording fluid upon arrival of the image forming apparatus, possibly resulting in a defective discharge operation.
In a conventional technology, the second liquid containing unit and the recording head may be filled with a fluid (which may be referred to as a “filling fluid” or “introductory fluid”) other than the recording fluid. Prior to starting a printing operation after arrival of the image forming apparatus, the filling fluid is suctioned out via a head nozzle surface and replaced with the recording fluid from the first liquid containing unit (see Patent Document 1, for example).
Specifically, the technology discussed in Patent Document 1 is directed to an ink filling method including the removal of bubbles in the inkjet head. Before filling the inkjet head with ink, the inkjet head is filled with a filling fluid having a higher viscosity than that of the ink in advance. The filling fluid used has a high viscosity ranging from 5 cPs to 50 cPs. It is discussed that the method prevents the generation of bubbles due to turbulence and the like when pouring the ink by using a purge unit (which is a maintenance unit) of the inkjet apparatus, thus enabling the gas/liquid substitution within the inkjet head without leaving bubbles. It is also discussed that the method is capable of substituting the filling fluid in the inkjet head with ink.
However, in a channel structure of the above second liquid containing unit, it is very difficult to substitute the filling fluid with ink when the air storing area of the second liquid containing unit is increased. Specifically, when the air storing area is large, stagnant areas with reduced flow speed may be present, resulting in a decrease in substitution efficiency. As a result, the drainage volume required for substitution increases, thus reducing efficiency. When the air storing area is provided in a negative pressure forming portion within the second liquid containing unit, the contained air may expand due to a change in temperature of the environment, resulting in a loss of the reduced-pressure state within the second liquid containing unit and possibly leading to a leakage of the ink from the head nozzle surface.
FIGS. 11A and 11B are a front view and a cross section, respectively, of a conventional head unit 55 including a sub-tank 15. The sub-tank 15 includes a fluid storing area 100 in which a negative-pressure may be formed and also a fluid can be stored, and an air storing area 101. The sub-tank 15 is connected to a recording head 14 via a filter 43. Within the sub-tank 15, a linked open/close valve 116 is installed as well known in the art and as described later with reference to FIG. 4. The fluid storing area 100 and the air storing area 101 are located on the side of the recording head with respect to the linked open/close valve 116; i.e., on the downstream side of the linked open/close valve 116. On the upstream side of the sub-tank 15, a fluid supply channel 16 and an ink tank 9 may be connected, as illustrated in FIG. 3.
The air storing area 101 of the sub-tank 15 provides a space for trapping bubbles that may enter through the fluid supply channel 16 or its various connecting portions during a long period of use. The air storing area 101 has a sufficiently large size so that the channel or the flow may not be closed or blocked by the air that floats and is collected at the top of the sub-tank 15. The sub-tank 15 and the recording head 14 are filled with a filling fluid in advance. The filling fluid may be substituted with ink by suctioning the filling fluid via a nozzle surface of the recording head 14 by using a cap (not illustrated) upon initial filling of the image forming apparatus upon delivery.
However, it has not been easy to completely substitute the filling fluid with ink because the upper portion of the air storing area 101 includes the stagnant area having a low flow speed, which prevents the easy mixing of the filling fluid with ink. In order to ensure the air storing area 101, the size of the sub-tank 15 may be increased. However, this results in an increase in the drainage volume for substituting the filling fluid with ink, resulting in a decrease in efficiency.
Further, because the air storing area 101 is communicated with the fluid storing area 100 including the negative-pressure forming unit, the stored air may expand due to a change in temperature of the environment, resulting in a loss of the reduced-pressure state within the fluid storing area 100 and potentially leading to the leakage of the ink via the head nozzle surface.
Patent Document 1 does not discuss these issues in connection with the structure of the liquid containing unit or the presence/absence of the air storing area, or does not even mention fluid substitution efficiency. Thus, the method according to Patent Document 1 is not capable of easily substituting the filling fluid with ink when the air storing area in the second liquid containing unit is increased.    Patent Document 1: Japanese Laid-open Patent Publication No. 2000-94708